The present invention relates generally to gas turbine engines, and, more specifically, to cooling therein.
In a turbofan gas turbine engine, air is pressurized in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases which flow downstream through several turbine stages which extract energy therefrom. The turbine stages include a row of nozzle stator vanes which turn and accelerate the combustion gases into a corresponding row of turbine rotor blades which extract energy therefrom for rotating a supporting disk. A high pressure turbine (HPT) first receives combustion gases from the combustor and powers the compressor. And, a low pressure turbine (LPT) is disposed downstream from the HPT and extracts additional energy for powering the fan upstream of the compressor which produces propulsion thrust for powering an aircraft in flight.
Since the turbine components are subject to heating by the combustion gases, the temperature thereof must be limited for obtaining a useful life during operation. And, temperature control of the turbine components is also required for reducing clearances between the stator and rotor components for maximizing the thermodynamic efficiency of the engine.
The various components of the HPT are typically cooled by using a portion of air bled from the compressor which is channeled through dedicated circuits specifically configured therefor. Since the LPT is disposed downstream from the HPT it typically does not require bleed air cooling.
However, a nacelle surrounds the core engine from the compressor to the LPT and typically includes an inlet at its forward end for receiving a small portion of the fan air for purging the bay defined between the nacelle and the various casings of the core engine. A purge air outlet is disposed near the aft end of the nacelle to drive the purge air at low velocity through the bay and remove any fuel vapors therein, as well as provide a small amount of cooling thereby.
The typical LPT casing is therefore subject to the purge air over its outer surface, and is not otherwise cooled. The turbine casing supports the various rows of nozzle vanes as well as turbine shrouds, both of which are maintained at suitable clearances or gaps with the cooperating rotor components. As the rotor and stator components expand and contract during operation, the clearances also expand and contract, and should be maintained as small as possible without experiencing undesirable rubbing between the stator and rotor components.
In some designs, turbine casings are externally cooled. For example, active clearance control is used for selectively cooling a turbine casing for in turn controlling the clearances for improving efficiency. However, these systems introduce additional complexity and expense in the engine.
Accordingly, it is desired to provide improved cooling of turbine casings and passive clearance control.